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Definition:
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Relatively strong winds concentrated within a narrow stream in the atmosphere. While this term may be applied to any such stream regardless of direction (including vertical), it is coming more and more to mean only a quasi-horizontal jet stream of maximum winds embedded in the midlatitude westerlies, and concentrated in the high troposphere. The question of the maintenance of the jet stream is a cardinal problem of theoretical meteorology. Two such jet streams are sometimes distinguished. The predominant one, the polar-front jet stream, is associated with the polar front of middle and upper-middle latitudes. Very loosely, it may be said to extend around the hemisphere, but, like the polar front, it is discontinuous and varies greatly from day to day. A subtropical jet stream is found, at some longitudes, between 20 and 30 latitude and is strongest off the Asian coast. Currently, in the analysis of upper-level charts, a jet stream is indicated wherever it is reliably determined that the wind speed equals or exceeds 50 knots.
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